In a color pair selecting section, a color pair which is used for 2-color printing is selectively inputted from a plurality of colors. In a print data separating section, print data is separated into data in different printing forms. In a print data developing section, one of the data in the different printing forms is color converted and developed by using one of the color pair and the other data in the different printing forms is color converted and developed by using the other one of the color pair. In an image drum up/down section, image drums to form images by colors other than the color pair are discriminated and held inoperative. Deterioration of expressing ability in the 2-color printing is prevented and a decrease in lives of the image drums which are not used is prevented.